In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor and fan. In a more complex version of the gas turbine engine, the compressor and a high pressure turbine are mounted on one shaft, and the fan and low pressure turbine are mounted on a separate shaft. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
Nickel-base superalloys are used in many of the highest-temperature materials applications in the aircraft gas turbine engine. For example, nickel-base superalloys are used to fabricate the high-pressure and low-pressure gas turbine blades. These turbine blades are subjected to extreme conditions of both stress and environmental conditions. The compositions of the nickel-base superalloys are engineered to carry the stresses imposed upon the blades. Protective coatings are typically applied to the gas turbine blades to protect them against environmental attack by the hot, corrosive combustion gases.
A widely used protective coating is an aluminum-containing coating termed a diffusion aluminide coating. An aluminum-containing layer is deposited upon the surface of the superalloy article. This aluminum-containing layer may include modifying elements such as platinum or palladium. During the deposition process and subsequently in service, the aluminum-containing layer interdiffuses with the substrate material and also oxidizes at its exposed surface to produce an aluminum oxide scale. After its initial formation, this aluminum oxide scale thickens only relatively slightly and remains highly adherent to the underlying material. The aluminum oxide scale thus protects the underlying aluminide coating and substrate against further oxidation and corrosion damage. Optionally, a ceramic thermal barrier coating may be applied overlying the aluminide coating and its aluminum oxide scale.
This approach to the formation of a protective coating is highly successful and widely used for many types of nickel-base superalloys. With other advanced superalloys, however, problems may arise. For example, rhenium is added to some nickel-base superalloys for improved mechanical properties. When such an enhanced-rhenium article is coated with an aluminide coating and then subjected to a sufficiently high temperature and long time at temperature, the aluminum of the coating chemically reacts with the rhenium and other constituents of the article substrate to form a secondary reaction zone (SRZ). The SRZ forms as acicular precipitates extending in a brittle layer inwardly into the article substrate for distances of up to about 0.010 inches, weakening the article substrate to that depth.
A substantial weakening of a depth of 0.010 inches of material may not be a concern in some applications. However, the gas turbine blades are usually hollow with typical wall thicknesses of 0.020-0.060 inches, or have other thickness dimensions within this range. A weakening of 0.010 inches of the depth of such a hollow gas turbine blade means that 15-50 percent of the wall thickness is weakened, greatly compromising the functionality of the turbine blade.
The problem of SRZ in aluminum-coated high-rhenium nickel-base superalloys has been recognized, and various techniques to avoid the formation of SRZ have been proposed. See, for example, U.S. Pat. Nos. 5,935,353; 5,334,263; and 5,598,968. These approaches are operable in many situations, but not in others.
There is accordingly a need for additional approaches for avoiding SRZ formation. The present invention fulfills this need in part, and further provides related advantages.